A waste heat steam generator is a heat exchanger which recovers heat from a stream of hot gas. Waste heat steam generators are typically used in combined-cycle gas and steam turbine power stations, in which the hot waste gases are conveyed to one or more gas turbines in a waste heat steam generator. The steam generated therein is subsequently used to drive a steam turbine. This combination produces electrical energy considerably more efficiently than gas or steam turbines alone.
Waste heat steam generators are able to be categorized on the basis of a plurality of criteria: Based on the direction of flow of the stream of gas, waste heat steam generators can for example be classified into vertical and horizontal designs. Furthermore steam generators exist with a plurality of pressure stages with different thermal states of the respective water-steam mixture contained therein.
Steam generators can generally be designed as natural circulation, forced circulation or continuous-flow steam generators. In a continuous-flow steam generator the heating of the evaporator tubes leads to a complete evaporation of the flow medium in the evaporator tubes in one pass. The flow medium—usually water—is fed after its evaporation to superheater tubes connected downstream from the evaporator tubes and superheated there. The position of the evaporation end point, i.e. the point of the transition from a flow with residual moisture to pure steam flow is variable and dependent on mode of operation in such cases. During full-load operation of such a continuous-flow steam generator the evaporation end point typically lies in an end area of the evaporator tubes, so that the superheating of the evaporated flow medium is already beginning in the evaporator tubes.
A continuous-flow steam generator, unlike a natural-flow or forced-flow steam generator, is not subject to any pressure restriction, so that it can be designed for fresh-steam pressures far above the critical pressure of water (pkrit≈221 bar)—in which water and steam cannot occur simultaneously at any temperature and thus no phase separation is possible either.
In order to guarantee secure cooling of the evaporator tubes, such a continuous-flow steam generator is usually operated in low-load mode or on start-up with a minimum flow of flow medium in the evaporator tubes. The minimum flow of flow medium provided for operation is thus not completely evaporated on starting up or in low-load mode in the evaporator tubes, so that with this type of operating mode, elements of unevaporated flow medium, i.e. a water-steam mixture, are still present at the end of the evaporator tubes.
Since the superheater tubes downstream from the evaporator tubes of the continuous-flow steam generator are usually not designed for a comparatively large throughflow of unevaporated flow medium, continuous-flow steam generators are usually designed such that, even on starting up and in low-load mode, a disproportionate entry of water into the superheater tubes is safely avoided. To this end the evaporator tubes are usually connected to their downstream superheater tubes via a water separation system. The water separator brings about a separation of the water-steam mixture escaping during start-up or in low-load mode from the evaporator tubes into water and steam. The steam is fed to the superheater tubes downstream of the water separation system whereas the separated water is typically fed back into the evaporator tubes via a circulation pump or can be drained away via an expansion unit.
The water separation system in this case can comprise a plurality of water separation elements which are integrated directly into the tubes. In such cases each of the parallel-connected evaporator tubes can especially be assigned a water separation element. The water separation elements can further be embodied as so-called T-piece water separation elements. Each T-piece water separation element in these cases comprises an inlet tube section connected to the upstream evaporator tube which, when seen in its longitudinal direction, extends into a water evacuation tube section, with an outflow tube section connected to the downstream superheater tube branching off in the transitional area.
This construction means that the T-piece water separation element is designed for inertial separation of the water-steam mixture flowing from the upstream evaporator tube into the inflow tube section. As a result of its comparatively high inertia, the proportion of water of the flow medium flowing into the inflow tube section at the transitional point namely preferably flows onwards in an axial extension of the inflow tube section and thus reaches the water evacuation tube section and from there usually flows on into a connected collection container. The steam component of the water-steam mixture flowing into the inflow tube section on the other hand, as a result of its comparatively small inertia, can better follow a forced redirection and thus flows via the water evacuation pipe to the downstream superheater tube section. A waste heat steam generator of this construction designed for continuous-flow mode is known for example from EP 1 701 090.
In a continuous-flow steam generator with a water separation system designed in this way, the local integration of the water separation into the individual tubes of the tube system of the continuous-flow steam generator means that the water can be separated without prior collection of the flow medium flowing out of the evaporator tubes. This means that a direct forwarding of the flow medium into an inlet collector of the downstream superheater tubes is also possible.
As a result of the construction the transfer of flow medium to the superheated tubes is additionally not just restricted to steam, instead a water-steam mixture can now also be passed on to the superheater tubes in that the water separation elements are oversupplied. This means that the evaporation end point can be relocated into the superheater tubes if required. This allows an especially high operational flexibility to be achieved, even on start-up or in low-load mode of the continuous-flow steam generator. In particular the fresh steam temperature can be regulated within comparatively wide limits by influencing the feed water quantity.
However account needs to be taken in such systems of the fact that, because the water separation function is already integrated into the individual tubes in the area of the separation system, a comparatively large number of individual tube sections or elements are required.